citra/dist/qt_themes/default/style.qss

QPushButton#GraphicsAPIStatusBarButton {
    color: #656565;
    border: 1px solid transparent;
    background-color: transparent;
    padding: 0px 3px 0px 3px;
    text-align: center;
    min-width: 60px;
    min-height: 20px;
}

QPushButton#GraphicsAPIStatusBarButton:hover {
    border: 1px solid #76797C;
}

QPushButton#3DOptionStatusBarButton {
    color: #A5A5A5;
    font-weight: bold;
    border: 1px solid transparent;
    background-color: transparent;
    padding: 0px 3px 0px 3px;
    text-align: center;
    min-width: 60px;
    min-height: 20px;
}

QPushButton#3DOptionStatusBarButton:hover {
    border: 1px solid #76797C;
}